Automatic gain control system



May 6, 1941- A. R. APPLEGARTH, JR 2,240,600

AUTOMATIC GAIN CQNTROL SYSTEM Filed July 16, 1958 3 Sheets-Sheet l W10/f M15 .L- L LL- l L ,VLLLLLL May 5, 1941 A. R. APPLEGARTH, JR 2,240,600

AUTOMATIC GAIN CONTROL SYSTEM May 6, 1941- A. R. APPLEGARTH, .1R 2,240,600

AUTOMATIC GAIN CONTROL SYSTEM Patented May 6, 1941 AUTOMATIC GAIN CONTROL SYSTEM Alexander R. Applegarth, Jr., Philadelphia, Pa., assigner, by mesne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa., a

corporation of Delaware Application July 16, 1938, Serial No. 219,643

8 Claims.

This invention relates to automatic means for controlling the gain of electrical systems which are designed to transmit signals containing a direct current component, such for example as those used in television receiving systems.

It is common practice, particularly in the radio art, to employ automatic volume control systems in which the derived control voltage is proportional to the average value of the received signal. Such systems function to maintain the average value of the incoming carrier signal substantially constant. This mode of operation is desirable in radio telephony since the average value of the transmitted carrier wave is constant and it is desired that the average value of the received carrier likewise be maintained constant in spite of variations in eld strength of the radiated Wave.

In the reception of television signals, however, a different situation obtains. Here it is desirable to vary the level of brightness at the receiver in accordance with variations in the brightness of the scene to be televised. In order to determine the brightness level it is customary to include in the transmitted signal a direct current component which varies in accordance with the brightness of the scene to be televised. In order to preserve this component it is necessary to employ direct-coupled or D. C. compensated amplifiers in the receiving system. Furthermore, it is not feasible to employ the usual type of automatic volume control system as used in receivers of audio modulated signals, since the effect would be to destroy the D. C. component and thus to prevent variation of the brightness level at the receiver in accordance with that of the scene televised.

It is the. object of this invention to provide novel means for controlling the gain of amplifying devices employed in receivers of signals transmitted by .means of modulated carrier waves. This novel means is characterized by its ability so to .modify the received signal as to maintain it of the same wave form as the transmitted signal regardless of any alterations which it may have undergone during transmission. In order to do this, it is necessary to preserve the D. C. component in the signal which cannot be done by using the usual automatic volume control means which, as has been observed, function to x the average value of received carrier.

In describing the nature and operation of the invention reference will be had to the accomy types of modulated carrier waves commonly used in television transmission;

Fig. 2 is a circuit diagram of a specific form of the invention;

Fig. 2a illustrates one method of applying the gain control voltage to an amplifier Whose gain is to be controlled.

Fig. 3 is a series of diagrams showing the mode of operation of the device of the invention;

Fig. 4 is an explanatory curve;

Figs. 5 and 6 are block diagrams showing the manners in which the device of the invention is applied to the conventional television receiving system; and

Fig. 7 is a circuit diagram of a `modified form of the invention.

As is well known, in the television art it is customary to transmit all of the essential intelligence in a single composite signal comprising tWo components which are established on opposite sides of a predetermined level which is initially xed. One of these components is characterized in that its average amplitude, referred to the predetermined level, inherently avries with time. This component is generally referred to as the video component, whose D. C. component varies with the picture background or average brightness. The instantaneous magnitude of this component, referred to the aforementioned predetermined level which is generally termed the blanking level, represents the brightness of one point or picture element of the scene to be televised. The signal component on the opposite side of the blanking level from the component whose average amplitude inherently varies, is one whose average amplitude is established at a constant value prior to the modulated carrier wave transmission of the composite signal. This component is generally referred to as. the synchronizing component. As a rule, it may comprise, at least in part, a series of time-spaced pulse signals occuring during intervals of greater duration than the pulse signals themselves and in which said i video signal component is established at said blanking level. The functioning of this component is to supply the necessary intelligence at the receiver for synchronizing the defiecting apparatus at the receiver with that at the transmitter, whereby the screen of the receiver cathode ray reproducing device is caused to be scanned at the same rate as that at the transmitter. Although the amplitude of one of these components is established at a constant amplitude prior to transmission, the amplitudes of both components may vary during transmission due to the extraneous influences operating upon the signal during such transmission.

A signal having the characteristics above dened corresponds to the envelopes of the modulated carrier wave signals represented in Figs. la and 1b. Fig. la shows a sample of what is generally referred to es a negatively modulated carrier wave while Fig. 1b shows a sample of a positively modulated carrier wave. The distinction lies solely in the relative positions of the two components of the composite signal envelope with reference to the zero carrier level. Thus it will be seen in Fi'g. 1a that the peaks of the synchronizing component correspond to the maximum values of carrier amplitude, while the maximum departures of the video component from the blanking level correspond to low carrier amplitudes. Since the blanking levelrepresents black in the pictures, in this case an increase in light in the scene televised will be represented by a decrease in the amplitude of the carrier in the periods between blanking intervals. In Fig. 1b, on the other hand, the synchronizing peaks correspond to a level in the vicinity of zero carrier while the maximum departures of the video component from the blanking level correspond to the carrier peaks so that in the case of positive modulation, increases'in light are represented by increases in carrier level.

Considering still the wave forms of Fig. 1 it will be seen that there are illustrated three different degrees of modulation corresponding respectively to normal light, greater than normal light, and less than normal light. It will be seen, therefore, that the average value of the video signal is not constant. This fact makes it difficult to devise a means for maintaining the received signal of the same wave-form and proportions as that transmitted, since it is not possible to use a control voltage proportional to the average value of the signal and obtain this result. The desired result is accomplished, according to the present invention, by the novel means now to be described. The invention makes use of the fact that in the composite signal, prior to its transmission, the blanking level and the amplitude of the synchronizing component are constant. In the received signal, however, they will not necessarily be constant but willvary in a manner depending on the conditions under which the signal is transmitted.

Fig. 2 shows a specific embodiment of the invention, the operation of which will now be described with the aid of Fig. 3. Referring to Fig. 3, (a) shows three time sections selected at several points on the time axis of a composite television signal such as might be obtained in the stage following the second detector in a television receiver. It will be observed that the first represents a signal of normal strength; the second, a weak signal in which the peak, blanking level, and background level are all lower than normal; and the third a strong signal in which the latter are all higher than those of the normal signal. It should be kept well in mind that these three portions of the signal are not successive but that they represent samples which are separated from one another by relatively long intervals of time, since it will be understood that automatic gain control systems do not operate instantaneously but rather tend to compensate for changes taking place over relatively long time intervals.

The first operation which may be performed on the demodulated signal is that of levelling the tips of the synchronizing pulses. This may be accomplished by the diode shown at V1 in Fig. 2 operating in conjunction with the condenser I and the resistor 2 in somewhat the same manner as a peak detector. The result of this operation is illustrated in Fig. 3--b and occurs in the following manner. The composite signal shown -in Fig. 3-a is introduced through the coupling condenser I and is impressed upon the plate of the diode across which is shunted the very large resistor 2. During lthe time that the signal is increasing, the condenser I will be charged rapidly through the diode resistance, and the point 3 will become negative with respect to ground. As the impressed voltage begins to decrease, the charge on the condenser will leak off slowly through the resistor 2 which is large by comparisonwith the resistance of thediode. Since the time constant of this circuit is high by comparison with the period of the input signal peaks, very little of the charge will have leaked of before the occurrence of another signal peak. Whenever the signal increases sufiiciently to exceed the voltage across the condenser, a charge will flow onto the plate of the diode. The result of the repetition of this action over a number of successive cycles is to establish a voltage at point 3 which has the effect of modifying the incoming signal so as substantially to level its upper peaks corresponding to the synchronizing pulses. The completeness of the levelling will of course depend on the rapidity with which the amplitude of the signal peaks is varying.

The levelled signal obtained in this manner is fed to the grid of the tube V2 as shown in Fig. 2. The voltage across resistor 2 serves to bias the grid of the tube V2 beyond cut-off and the plate voltage of this tube is so adjusted that only the desired upper portion of the levelled signal is passed. The output of this tube produces an amplified voltage across the cathode resistor 1, which is integrated by a filter comprising the condensers 4 and 5 and the resistor 6. The averaged and amplified voltage may then be fed back to a previous stage as an automatic gain control voltage. The use of a cathode resistor 1, instead of the usual plate load resistor, is preferable in order to obtain the desired polarity of control voltage variation, as will become apparent as the description proceeds but does not affect the amplifying properties of the tube. The control voltage may be applied to the amplifier whose gain is to be controlled employing any of the conventional methods which are familiar to those skilled in the art. A suitable method is shown in Fig. 2a where Vs represents a tube functioning as a part of an intermediate frequency amplifying stage. The control voltage may be applied to the control grid of this tube through the resistor Il,

, by-passed by the condenser I8. It will, of course,

be recognized that the invention is not restricted to this particular mode of applying the control voltage and that any well known method may be employed, and the control voltage applied to any desired number of stages.

'I'he satisfactory operation of the device depends primarily upon the adjustment of the bias of the tube V2. If, for example, the tube be biased so as to pass only that portion of the leveled signal above the line A in Fig. 3, the output current will be of the form shown in (c). The pulses are of the same size for all of the three cases, and since the magnitude of the control may be considered proportional to the difference in area be tween the successive pulses, there will be no conm1. on the other hand, if the cube be so biased that all of the leveled signal above the line B is passed, then the output will be of the form shown in (d). In this case, a weak signal gives rise to a pulse which is larger than that corresponding to the normal signal by the amount represented by the shaded area in thegure, and a strong signal gives rise to one which is correspondingly smaller as shown. It will, therefore, be seen that it is desirable to choose the bias voltage so as normally to transmit a small portion of the com-l posite signal below the blanking level but not s much that the control voltage will be influenced by the video background, More specifically it will be desirable to adjust the bias voltage so that the average value of the blanking level just exceeds the level of selection in the tube.

The behavior of the system for different settings of the cut-off level may more clearly be shown by the curve of Fig. 4 where control voltage is plotted against the plate voltage of the automatic gain control tube V2. This is equivalent to a plot of control voltage versus cut-off level since the cut-off level will vary with the plate voltage. An operating point A' on the horizontal portion of the curve corresponds to location of the cut-off above the blanking level as at A in Fig. 3-b, while an operating point B' on the sloping portion correspondsto a cut-off level below the blanking level as at B in Fig. 3-b. Variations in the amplitude of the signal will have the eiect of producing excursions along the curve to either side of the operating point. If the point is at A', no control voltage will result for small excursions to either side and the gain will not be aiected. If it is located at B', as it is in practice, control voltage will be developed which will vary the gain in the desired manner. Of course the further from the bend of the curve the operating point is located, the better will be the control for increases in signal amplitude. However itmust also be observed that the further from the bend the operating point is placed, the more opportunity there is for the video and background component to be included in the control voltage, and hence the greater the tendency to cause the average value of the video signal to be constant. Since this is not desired, it is customary in practicing the invention to operate as near the bend of the curve as is consistent with obtaining complete control.

The operating point is determined by the adjustment of the voltages applied to the tube V2. This is accomplished as shown in Fig. 2 by adjusting the potentiometers 8 and 9. Potentiometer 8 is first adjusted so that when the tube V2 is cut off the I. F. amplier is also biased to cutoff. Potentiometer 9 isthen adjusted to establish the cut-off point of the tube V2, or, in short, to set the above-mentioned operating point.

In the above discussion, the device has been characterized as a means for maintaining the amplitude of the synchronizing pulses constant. It will be understood, however, that in achieving this, the video signal willalso have been corrected to its original waveform, since it can be assumed that any amplitude distortion of the synchronizing signal caused by variations in field strength in the transmitted wave would have produced proportionate amplitude -distortion in the video and background signal. Hence, if one component of the signal is corrected and the others are corrected proportionately thereto the result will be a composite signal which is a duplicate in waveform of the original signal.

Signal may be supplied to the levelling'tube from any source of demodulated composite television signal such, for example, as the second detector output or one of the stages following it. This applies whether the signal was obtained by demodulation of a positively or a negatively modulated carrier, since in either case the output may be levelled along the synchronizing peaks. This method of connection is shown in Fig. 5.

Another method of applying the device is to supply it with negatively modulated carrier from some point in advance of the second detector. Because of the large time constant of the levelling circuit, the device will not tend to follow the rapid alternations of the carrier frequency but rather will follow the slower variation in the amplitude of the synchronizing peaks when the carrier is modulated in the negative manner as shown in Fig. l. Thus in the case of negative modulation the result will be the same as if demodulated signal had been supplied to the levelling diode, and the manner of connecting the device for such operation is shownin Fig. 6. However, if positive modulation has been employed, this method is no longer applicable, as will readily be seen, for the control voltage would tend to follow the carrier peaks which vary with the background as shown in Fig. 1. Since this is not desired, it is apparent that, when positive modulation has been employed, the carrier signal should first be demodulated according to the method of Fig. 5.

In some instances it may be desirable to obtain a larger control voltage than can be obtained by using the system of Fig. 2. This can be done by amplifying the control voltage by means of a D. C. or other suitable amplifier, as shown in Fig. 7. In this gure, V3 is the automatic gain control tube, I0 is a load resistor which coacts with the condenser II to filter the control voltage, V4 is the amplifier tube, and I2 is its load resistor which likewise coacts with the condenser I3 further to filter the control voltage. The potentiometer I4 is adjusted so as to place the lower end of the load resistor I2 at a small negative potential with respect to ground. This has the effect of maintaining a small negative bias upon the grids of the controlled tubes at all times which is necessary for stable operation. The potentiometer I5 is used to adjust the operating conditions of the system, so that there will be a. small delay in the operation of the system, which also is desirable to insure stability.

It will be observed that the levelling diode has been omitted in this embodiment, its function being performed by the grid and cathode of the triode V3 operating as a diode and coacting with the high resistance IE in the manner hereinbefore described.

It will be' seen that the device of Fig. 7 is essentially that of Fig. 2 with the addition of the D. C. ampliiier. The operation of the device will be understood from the foregoing description. The control voltage may be applied to one or more stages of the signal channel in any suitable manner, for example in the manner illustrated in Fig. 2a. l

To disclose and explain the principles of this invention, examples have been chosen which show its application in the television art, since it is in that eld that the device has found its principal usefulness. It will be understood, however, that the invention is not restricted to use in television systems but may be applied to any electrical system in which it is desired to use automatic gain control which is responsive to only a certain portion of the signal but not to variations in the average value thereof, Furthermore, the invention is not restricted to use with the particular types of signal here shown but may be used with any signal from which it is possible to derive avarying control voltage in the manner herein disclosed. Various embodiments and modiflcations of the invention are therefore readily possible beyond the forms illustrated and described herein.

I claim:

l. In a composite signal system having a signal channel adapted to transmit a composite signal comprising a first component whose average amplitude inherently varies, and a second component whose average amplitude may vary due to extraneous iniiuences, said components being established on opposite sides of a predetermined level, apparatus for controlling the gain of said signal channel, comprising means for deriving a composite signal from said system, means for establishing a selection level determined with reference to the peaks of said second component so as to define a portion of said signal which normally includes said second component and a part of said first component, said part being small by comparison with the whole of said first component, whereby the portion thus defined is substantially independent of variations in said first component, means for selecting said dened portion of the signal, means for deriving a control voltage whose magnitude Varies according to the variations in said selected portion, and means for applying said control voltage to said signal channel in a manner to control the gain thereof.

2. In a composite signal system having a signal channel adapted to transmit a composite signal comprising a iirst component whose average amplitude inherently varies, and a second component whose average amplitude may vary due to extraneous influences, said components being established on opposite sides of a predetermined level, apparatus for controlling the gain of said signal channel, comprising means for deriving a composite signal from said system, means for modifying said derived signal so as to bring the peaks of said second component to a substantially common level, means for establishing a selection level fixed with reference to the peaks of said second component so as to define a portion of said signal which normally includes said second component and a part of said first component, said part being small by comparison with the whole of said first component, whereby the portion thus defined is substantially independent of variations in said iirst component, means for selecting said defined portion of the signal, means for deriving a control voltage whose magnitude varies according to the variations in said selected portion, and means for applying said control voltage to said signal channel in a manner to control the gain thereof. y

3. In a composite signal system having a signal channel adapted to transmit a composite signal comprising a first component whose average amplitude inherently varies, and a second component whose average amplitude may vary due to extraneous influences, said components being estabilshed on opposite sides of a predetermined level, and said first component having a relatively smaller energy content than said second component, apparatus for controlling the gain of said signal channel, comprising means for deriving a composite signal from said system, means for establishing a selection level determined with reference to the peaks-of said second component so as to define a portion of said signal which normally includes said second component and a part of said first component, said part being small by comparison with the whole of said first component, whereby the portion thus dened is substantially independent of variations in said first component, means for selecting said defined portion of the signal, means for deriving a control voltage whose magnitude varies according to the energy variations in said selected portion, and means for applying said control voltage to said signal channel in a manner to control the gain thereof.

4. In a composite signal system having a signal channel adapted to transmit a composite signal comprising a first component whose average amplitude inherently varies, and a second component whose average amplitude may vary due to extraneous influences, said. components being established on opposite sides of a predetermined level, apparatus for controlling the gain of said signal channel, comprising means for deriving a composite signal from said system, means for establishing a selection level determined with reference to the peaks of said second component so as to define a portion of said signal which normally includes said second component and a part of said first component, Said part being small by comparison with the whole of said first component, whereby the portion thus defined is substantially independent of variations in said first component, means for selecting said defined portion of the signal, integrating means for averaging said selected portion, and a connection for supplying said averaged portion to said signal channel in a manner to control the gain thereof.

5. In a composite signal system adapted to transmit a composite signal comprising a first component whose average amplitude inherently varies, and a second component whose average amplitude may varyl due to extraneous influences, said components being established on opposite sides of a predetermined level, apparatus for controlling the gain of said system, comprising means for deriving a composite signal from said system, means for establishing upper and lower levels of selection determined with reference to the peaks of said second component so as to de- .flne a portion of said signal between said levels which normally includes parts of both of said components, the part k of said first component being small by comparison with the whole of said first component, whereby the portion thus defined is substantially independent of variations in said rst component, means for selecting said defined portion ofthe signal, means for deriving a control voltage whose amplitude varies according to the variations in Said selected portion, and means for applying said control voltage to the signal channel of the systemvin a manner to control the gain thereof.

6. In a television system employing a composite signal including video and synchronizing components established on opposite sides of a blanking level, a device for controlling the gain of the system, comprising means for deriving a composite signal from the system, means for estab- -lishing a selection level determined with reference to the peaks of said synchronizing component so as to define a portion of Said derived signal which normally includes said synchronizing component and a part of said video component, said part being small by comparison with the whole video component, whereby the portion thus deiined is substantially independent of variations in said video component, means for selecting said defined portion of the signal, means for deriving a control voltage whose amplitude varies according to the variations in said selected portion, and means for applying` said control voltage to the signal channel of the system in a manner to control the gain thereof.

7. In a television system employing a composite signal including video and synchronizing components established on opposite sides of a blanking level, a device for controlling the gain of the system, comprising means for deriving a composite signal from the system, means for modify, ing said derived signal so as to bring the peaks of said synchronizing component to a substantially common level, means for establishing a selection level xed with reference to the peaks of said synchronizing component so as to denne a portion of said derived signal which normally includes said synchronizing component and a part of said video component, said part being small by comparison with the whole video component, whereby the portion thus defined is substantially independent of variations in said video component, means for selecting said deilned portion of the signal, means for deriving a control voltage whose amplitude varies according to the variations in said selected portion, and means for applying said control voltage to the signal channel of the system in a manner to control the gain thereof.

8. In a television system, a source of a composite television signal comprising video and synchronlzing components established on opposite sides of a blanking level, said synchronizing component comprising a series of time-spaced pulse signals occurring during intervals of duration greater than said pulse signals and in which said video signal component is established at said blanking level, an ampller for said signal and means for controlling the gain of said amplier, including a space discharge device having an anode, a cathode, and a control grid, a resistor shunted diode, said diode having its anode coupled to the grid of said space discharge device and its cathode coupled to the cathode oi said space discharge device, means for deriving a signal from said ampliiier, means comprising a condenser for supplying said derived signal to the grid of saidspace discharge device, the time constant of the combination comprising said re sistor and said condenser being chosen relatively large by comparison with the interval between said synchronizing pulses, and the ratio of resistance to capacitance being chosen so as to develop a bias of such magnitude that said space discharge device is normally operative to transmit said synchronizing component and a part of said video component, said part being small by comparison with the whole of said video component whereby the portion transmitted is substantially independent of variations in said video component, means associated with the anode circuit of said space discharge device for integrating the current therein to develop a control voltage, and means for applying said control voltage to said ampliiler to control the gain thereof.

ALEXANDER R. APPLEGARTH, J R. 

